Method, system, and device for differential current injection

Abstract

A laser diode, comprising: a transverse waveguide including an active layer located between an n-type semiconductor layer and a p-type semiconductor layer, wherein the transverse waveguide is bounded on the n-side of the transverse waveguide by a lower-index n-cladding layer and on the p-side of the transverse waveguide by a lower-index p-cladding layer; a cavity orthogonal to the transverse waveguide, wherein the cavity is bounded at a first end by a high reflector (HR) facet in a longitudinal direction and at a second end by a partial reflector (PR) facet; and a first contact layer electrically connected to the waveguide and configured to vary the amount of current injected into the waveguide in the longitudinal direction so as to inject more current near the HR facet than at the PR facet.

Description

Technical Field

[0001] The technology disclosed herein relates to diode lasers, and more particularly to methods, systems, and devices for tuning current injection in a waveguide of a diode laser. Background Art

[0002] A laser is a light-emitting device. Light emission in a laser is the result of the optical amplification of stimulated emission of electromagnetic radiation. Some lasers emit spatially and temporally coherent light, which allows the laser to emit light with a narrow optical bandwidth that can be narrowly focused over long distances. There are many types of lasers, such as gas lasers, chemical lasers, dye lasers, metal vapor lasers, solid-state lasers, and semiconductor lasers.

[0003] A laser diode is an electrically pumped semiconductor laser formed by growing multiple layers of semiconductor materials on a suitable substrate, the substrate having a lattice constant that allows selection of materials to produce a desired emission wavelength. A laser diode typically includes an active layer disposed between a p-type semiconductor material layer and an n-type semiconductor material layer. Many laser diodes are fabricated on semiconductor substrates such as gallium arsenide (GaAs), doped with elements such as aluminum (Al), silicon (Si), zinc (Zn), carbon (C), or selenium (Se) to produce n- and p-type semiconductor layers. The active layer is typically undoped gallium indium arsenide (GaInAs), which may be only a few nanometers thick. In a typical laser diode, when a forward bias is applied, electrons and holes recombine in the active layer to produce light. The active layer (e.g., quantum well, quantum wire, or quantum dot, type-II quantum well) is located in a waveguide layer, which has a higher refractive index compared to the surrounding p- and n-doped cladding layers. The light generated from the active layer is substantially confined (or "guided") within the plane of the waveguide.

[0004] Some edge-emitting Fabry-Perot diodes of broad area lasers (BALs) are arranged in a rectangular gain or index-guided semiconductor structure; other BALs, such as reduced mode (REM) devices, are arranged to have a flared laser oscillator waveguide as described in U.S. Patent No. 9,166,369. In BALs and REM devices, the opposing end faces of the waveguide define a high reflector and a partial reflector, thereby providing feedback for optical oscillation within the resonator. The longitudinal distance between the two opposing end faces is referred to herein as the "length" of the waveguide, lateral cavity, cavity, waveguide cavity, and / or laser. The multi-layer laser diode structure extends the length of the laser and has a wide width for electrical injection that extends to opposing side surfaces that also extend the length of the laser. The multi-layer semiconductor material is typically arranged such that the laser operates in a single mode along the growth direction of the laser. This direction is defined as the fast axis direction. Since the semiconductor laser operates in a single mode along the fast axis direction, it is not possible to further increase the brightness of the laser diode in this direction - the so-called diffraction limit. The thickness or distance between the top and bottom surfaces of the multi-layer semiconductor laser structure provides a smaller end facet size. This "stripe thickness" is typically on the order of a few micrometers. The width of the multi-layer laser structure provides a larger end facet size. The "stripe width" is typically on the order of tens to hundreds of micrometers and is referred to as the "slow axis". Since the stripe width is much larger than the wavelength of light, the lateral characteristics of the optical field propagating along the waveguide optical axis are highly multimode. The slow axis divergence angle is much smaller than the fast axis divergence angle.

[0005] BALs are used for high power applications. Since BALs have multiple modes on the slow axis, their slow axis beam parameter product (BPP) tends to be higher than that of single mode laser diodes. Additionally, when they are driven to higher currents, the thermal lensing effect becomes more pronounced, which results in a higher index contrast distribution in the lateral direction, leading to accommodation of even more modes. Therefore, as the lateral divergence angle widens, this results in a decrease in the lateral beam parameter product (BPP) and brightness (power ÷ BPP).

[0006] For power scaling applications and reducing the cost per watt of manufacturing diode lasers, it is highly desirable to have higher brightness for each emitter at higher output powers. The brightness of BALs and REM devices can be increased by driving to higher currents; however, the current at which maximum brightness occurs also occurs at progressively lower current values. Therefore, the maximum output power at maximum brightness also decreases.

[0007] "Slope efficiency" is a metric obtained by plotting the laser output power against the input pump power. The slope efficiency is the slope of this line. When the diode laser is driven to high currents, the slope efficiency not only begins to roll over but also begins to decline. This decline becomes more prominent as the cavity length of the diode laser increases. For a given cavity length, this is also more pronounced for wider emitter devices, as shown in Figure 1 as shown. These observations suggest thermal losses due to lateral thermal diffusion reducing the slope efficiency.

[0008] In addition, when comparing the output power with the current density, a reduction (i.e., a decline) in slope efficiency is seen at lower current densities. Figure 2 L-I plots of 75μm and 150μm 5mm and 8mm BAL versus current density (equivalent current) are shown, showing the L-I decline that occurs at lower current densities for longer cavities. The area formed by the product of 75μm x 5mm is considered the unit area of this plot. Typically, there is a longitudinal temperature gradient along the cavity. The output (partial reflector (PR)) facet end operates hotter compared to the back (high reflector (HR)) facet end. The root cause of this is the non-uniform waste heat that is nearly exponentially correlated with the optical intensity along the cavity direction, and the initial asymmetric thermal boundary conditions, both of which allow for more efficient cooling on the high reflector side of the BAL.

[0009] At high drive currents, the longitudinally varying waste heat and power conversion result in a gradually increasing temperature difference from the output (PR) facet to the back (HR) facet. Consequently, the thermionic emission that ultimately leads to carrier leakage and loss also has a longitudinal dependence. More thermionic emission occurs at the front compared to the back of the laser. This is clearly seen from the broadening of the spectral width as a Figure 3 and Figure 4 function of the drive current as shown in Figure 3 The spectral widths (proportional only to the cavity length) of an 8mm×95μm (red) device and a 5mm×95μm (blue) device are shown as a function of the linear current density. Figure 4 The spectral widths of an 8mm×95μm (red) device and a 5mm×95μm (blue) device are shown as a function of the diode "canonical temperature", as determined by the centroid of the spectral width shift with current, and assuming a calibration coefficient of 0.32nm / C for GaAs / InGaAs. As shown, at the same junction temperature, the spectral emission width of the 8mm cavity length device is wider than that of the 5mm cavity length device. This indicates a larger temperature gradient in the longer cavity device.

[0010] There is also evidence that the drop is not due to longitudinal spatial hole burning (LSHB). Figure 5 The L-I for pulses up to >40 A (5 μs 1% duty cycle) is shown, showing no power drop, which would occur if LSHB were a contributor. In fact, Figure 5 an L-I curve that is nearly linear in the same current range is shown. Under such pulse conditions, LSHB would result in a drop in the L-I curve.

[0011] The above-described drop in slope efficiency at higher current densities is at least partially due to lateral cavity heat diffusion, which reduces the ability to extract higher power from the laser diode at high current densities, especially for longer cavity devices. In addition, near the diode output facet, the facet temperature increases and the quantum well bandgap decreases, thereby having an adverse effect on reliability. What is needed is a method and / or device for improving these negative effects on slope efficiency when driving the BAL to higher currents to reduce lateral cavity heat diffusion, which appears to contribute to the drop in slope efficiency. SUMMARY OF THE INVENTION

[0012] Disclosed herein is a laser diode that includes: a transverse waveguide that includes an active layer between an n-type semiconductor layer on the n-side of the transverse waveguide and a p-type semiconductor layer on the p-side of the transverse waveguide, wherein the transverse waveguide is bounded on the n-side by a lower-index n-cladding layer and on the p-side by a lower-index p-cladding layer; a cavity that is orthogonal to the transverse waveguide, wherein the cavity is bounded in the longitudinal direction at a first end by a high reflector facet and at a second end by a partial reflector facet; and a first contact layer that is configured to vary the amount of current injected into the cavity in the longitudinal direction such that more current is injected at the first end than at the second end.

[0013] The laser diode can be configured such that a first contact layer is disposed on the n-side of the transverse waveguide. The laser diode can be configured such that a first contact layer is disposed on the p-side of the transverse waveguide. The laser diode can be configured such that the first contact layer includes a substantially uniform thickness. The laser diode can be configured such that the first contact layer includes a material thickness gradient that increases longitudinally from a first end to a second end. The laser diode can be configured such that the first contact layer includes a resistance gradient that increases longitudinally from a first end to a second end. The laser diode can be configured such that the first contact layer includes a contact material surface area gradient that decreases longitudinally from a first end to a second end. The laser diode can be configured such that the first contact layer includes a dopant. The laser diode can be configured such that the first contact layer is highly doped. The laser diode can be configured such that the n-cladding layer below the first contact layer is moderately doped. The laser diode can be configured such that the p-cladding layer below the first contact layer is moderately doped. The laser diode can be configured such that the contact material surface area gradient includes a contact pattern. The laser diode can be configured such that the contact pattern includes a plurality of discrete contacts and is configured to grade current injection by a change in contact surface area size between different ones of the plurality of discrete contacts, wherein the surface area of the discrete contacts decreases longitudinally from the first end to the second end. The laser diode can be configured such that the contact pattern includes a continuous shape configured to gradually decrease the contact material surface area distribution longitudinally from the first end to the second end. The laser diode can be configured such that the first contact layer includes a metal alloy having reduced conductive properties. The laser diode can further include a second contact layer disposed opposite the first contact layer and including a different contact pattern.

[0014] The present disclosure further provides a laser diode, comprising: a transverse waveguide including an active layer between an n-type semiconductor layer on the n-side of the transverse waveguide and a p-type semiconductor layer on the p-side of the transverse waveguide, wherein the transverse waveguide is bounded on the n-side by a lower-index n-cladding layer and on the p-side by a lower-index p-cladding layer; a cavity orthogonal to the transverse waveguide, wherein the cavity is bounded at a first end by a high reflector facet and at a second end by a partial reflector facet in a longitudinal direction; a first p-side contact and a second p-side contact, both electrically isolated from each other, wherein the first p-side contact and the second p-side contact are electrically connected to the cavity and an n-side contact, wherein the first p-side contact is configured to inject a first current density into the cavity, and the second p-side contact is configured to inject a second current density into the cavity.

[0015] The laser diode can be configured such that the first current density and the second current density are different. The laser diode can be configured such that the first p-side contact and the second p-side contact are uniformly distributed along the cavity, wherein the first p-side contact is disposed close to the first end, the second p-side contact is disposed close to the second end, and the first current density is greater than the second current density.

[0016] The laser diode can be configured such that the first p-side contact and the second p-side contact are arranged closer to the first end than the second end, such that when active, more current is injected into the cavity at a location closer to the first end compared to the second end. The laser diode can further include a third p-side contact and a fourth p-side contact that are electrically isolated from each other and electrically coupled to the cavity and the n-side contact. The laser diode can be configured such that the first p-side contact and the second p-side contact are coupled to respective ones of a first heat sink base and a second heat sink base. The laser diode can be configured such that the first p-side contact and the second p-side contact have different thicknesses. The laser diode can be configured such that the first p-side contact and the second p-side contact include materials having different resistance characteristics. Description of the Drawings

[0017] The drawings, in which like reference numerals refer to like elements, are incorporated in and constitute a part of this specification, and together with the description, explain the advantages and principles of the disclosed technology. In the drawings,

[0018] Figure 1 show the L-I graphs of 75μm and 150μm 5mm and 8mm BAL versus current;

[0019] Figure 2 show the L-I graphs of 75μm and 150μm 5mm and 8mm BAL versus current density;

[0020] Figure 3 show the spectral widths of 8mm×95μm and 5mm×95μm as a function of linear current density;

[0021] Figure 4 show the spectral widths of 8mm×95μm and 5mm×95μm as a function of the "specification temperature" of the diode, i.e., the junction temperature;

[0022] Figure 5 show the pulsed L-I graphs up to >40A;

[0023] Figure 6 show a cross-sectional perspective view of an exemplary vertical epitaxial layer structure including a differential current injection contact layer;

[0024] Figure 7A show a cross-sectional perspective view of an exemplary vertical epitaxial layer structure including a differential current injection contact layer;

[0025] Figure 7B show an example of a surface area contact for a vertical epitaxial layer structure including a differential current injection contact layer;

[0026] Figure 8Ais a cross-sectional perspective view showing a vertical epitaxial layer structure including an n-type differential current injection contact layer;

[0027] Figure 8B is a plan view showing a laser diode n-contact pattern of a differential current injection contact layer;

[0028] Figure 9A is a cross-sectional view showing a vertical epitaxial layer structure including a plurality of electrically isolated p-contacts configured to vary the current density along a longitudinal cavity; and

[0029] Figure 9B shows an example of electrically isolated p-contacts having different thicknesses. DETAILED DESCRIPTION

[0030] As used in this application and the claims, the singular forms "a", "an", and "the" include plural forms unless the context clearly dictates otherwise. Additionally, the term "comprising..." means "including...". Further, the term "coupled" does not exclude the presence of intermediate elements between the coupled items.

[0031] The systems, devices, and methods described herein should not be construed as being limited in any way. Instead, the present disclosure relates to all novel and non-obvious (both individually and in various combinations and sub-combinations with each other) features and aspects of the various disclosed embodiments. The disclosed systems, methods, and devices are not limited to any particular aspect or feature or combination thereof, and the disclosed systems, methods, and devices do not require the presence of any one or more particular advantages or problems to be solved. Any theory of operation is for the purpose of facilitating explanation, but the disclosed systems, methods, and devices are not limited to such a theory of operation.

[0032] Although the operations of some of the disclosed methods are described in a particular, sequential order for convenience of presentation, it should be understood that such description encompasses rearrangements, unless the specific language set forth below requires a particular ordering. For example, the operations described in sequential order may in some cases be rearranged or performed concurrently. Additionally, for simplicity, the figures may not show the various ways in which the disclosed systems, methods, and devices may be used in conjunction with other systems, methods, and devices. Further, the specification sometimes uses terms such as "produce" and "provide" to describe the disclosed technology. These terms are high-level abstractions of the actual operations performed. The actual operations corresponding to these terms will vary depending on the particular embodiment and will be readily discernible to those skilled in the art.

[0033] In some examples, values, procedures, or devices may be referred to as "lowest", "best", "minimum", etc. It should be understood that such descriptions are intended to indicate that a choice can be made among many alternative functions that are used, and that such choice does not need to be better, smaller, or otherwise more preferred than other choices. Examples are described with reference to directions indicated as "above", "below", "upper", "lower", etc. These terms are used for convenience of description but do not imply any particular spatial direction.

[0034] For purposes of simplicity and illustration, Figure 6 Figs. 5 to 9 illustrate examples of a differential current injection contact layer (DCICL) that is configured to differentially inject current along a lateral cavity portion of a quantum well laser. However, various other types of lasers can be configured to include a DCICL. For example, a DCICL can be formed on a double heterostructure laser, an interband cascade laser, a distributed Bragg reflector laser, a distributed feedback laser, a quantum cascade laser, a vertical cavity surface emitting laser, and / or a vertical external cavity surface emitting laser, etc. Thus, the claimed subject matter is not intended to be limited to quantum well laser diodes.

[0035] Methods and apparatus are disclosed herein for ameliorating the adverse effects of driving a BAL to higher currents (e.g., greater than about 10 A), and in particular for reducing the degradation of slope efficiency. These methods and apparatus are directed to reducing lateral cavity thermal spreading in a laser diode, which contributes to the rollover and degradation of slope efficiency. These methods include pumping the laser diode at different current densities along the cavity length to reduce the longitudinal thermal gradient and prevent or mitigate rollover and degradation of L-I in all cases, but particularly for longer cavity length laser devices where the cavity length is greater than about 5.0 mm long. The most effective method is to pump the cavity with a continuously variable current density; the simplest method is to divide the cavity into two or more appropriate front and back segments of lengths, and differentially pump these two or more segments at a higher current density near the rear high reflector end than at the front partial reflector end. This can also reduce the facet thermal load and local temperature on the facets. Additionally, it can mitigate the effect of bandgap shrinkage on the partial reflector facet as compared to a uniform pumping arrangement.

[0036] Figure 6A cross-sectional perspective view of a vertical epitaxial layer structure of an example laser diode 600 including a differential current injection contact layer (DCICL) 614 is shown. In the example, the laser diode 600 is formed to include a substrate 604 (shown in a broken-away manner in width), an n-type semiconductor layer 606, and a p-type semiconductor layer 608. An active layer (quantum well 602) is located between the n-type semiconductor layer 606 and the p-type semiconductor layer 608. An n-cladding layer 610 is provided outside the n-type semiconductor layer 606 and has a lower refractive index than the n-type semiconductor layer 606. A p-cladding layer 612 is provided outside the p-type semiconductor layer 608 and has a lower refractive index than the p-type semiconductor layer 608. The quantum well 602, the n-type semiconductor layer 606, and the p-type semiconductor layer 608 constitute a portion of a transverse waveguide 618 of the laser diode 600. The transverse waveguide 618 is bounded on the n-side 624 in the Y direction by the lower-index n-cladding layer 610 and on the p-side 626 by the lower-index p-cladding layer 612. The boundaries of the n-side 625 and the p-side 626 are identified by brackets.

[0037] In the current example, the DCICL 614 is a p-contact layer and is provided on the p-cladding layer 612. An N-contact 616 is provided on the substrate 604. A cavity 640 defined by the DCICL 614 is orthogonal to the transverse waveguide 618 and is bounded at a first end 520 in the longitudinal or Z direction by a high reflector (HR) facet 642 and at a second end 622 by a partial reflector (PR) facet (not shown).

[0038] In the example, the p-contact of the DCICL 614 is configured to vary the amount of current injected into the injection cavity 640 in the longitudinal direction, thereby creating a current injection gradient. The current injection can be varied to achieve the current injection gradient by changing the resistance of the DCICL 614. The resistance can be longitudinally graded in several ways. For example, the volume of material through which current must pass can be varied by grading the thickness of the DCICL 614 in the longitudinal direction. The material thickness T1 at the first end 620 is less than the material thickness T2 at the second end 622. The material thickness of the DCICL 614 can gradually increase from the first end 620 to the second end 622 to create a thickness gradient. The resistance near the second end 622 is higher than that near the first end 620, and the voltage drop along the cavity 640 in contact with the p-cladding layer will change, whereby the injected current can be varied. The current injection gradually increases from the lowest current injection amount near the PR facet at the second end 622 to the highest current injection amount near the HR facet at the first end 620. When the diode 600 is operating, the second end 622 near the PR facet tends to operate at a higher temperature than the first end 520 near the HR facet 642. By varying the current injection such that less current is injected near the PR facet (at the end 622) than near the HR facet 642 (at the end 620), the longitudinal thermal gradient can be reduced, which can prevent or mitigate the rollover and droop of L-I in all cases, but especially for laser devices with longer cavity lengths, where the cavity length is greater than about 5.0 mm in length. In some examples, the current injection gradient as described herein can increase the current injection density from the second end 622 to the first end 620 by an order of magnitude of 0.001%, 1%, 5%, 10%, 50%, 100%, or >100%. In other examples, the current injection gradient as described herein can increase the current injection density from the second end 622 to the first end 620 to an amount sufficient to reduce the longitudinal thermal gradient in order to mitigate or prevent the reduction in slope efficiency that occurs when the BAL is driven to higher currents. In this way, the adverse effects (e.g., reduction in slope efficiency) of driving the BAL to higher currents (e.g., greater than about 10 A) can be reduced.

[0039] The p-contact material of the DCICL 614 can include any of a variety of electrical conductors such as titanium (Ti), aluminum (Al), gold (Au), nickel (Ni), platinum (Pt), gallium nitride (GaN), gallium indium nitride (GaInN), and / or the like or any combination thereof. In some examples, metal alloys with reduced conductive properties such as titanium nitride (TiN), low temperature growth (LTG) gallium arsenide (GaAs), etc. or any combination thereof can be used to fabricate the DCICL 614 to achieve the desired resistance change. In certain embodiments, the term "reduced conductive properties" can refer to conductive properties that are lower compared to the conductivity of p++GaAs semimetal.

[0040] Figure 7A is a cross-sectional perspective view showing a vertical epitaxial layer structure of an exemplary laser diode 700 including a DCICL 714. In this example, the longitudinal p-contact DCICL 714 pattern is configured to grade current injection by varying the contact size. Similar to Figure 6 the laser diode 600 shown in, the laser diode 700 is formed to include a substrate 704, an n-type semiconductor layer 706, and a p-type semiconductor layer 708. An active layer (quantum well 702) is disposed between the n-type semiconductor layer 706 and the p-type semiconductor layer 708. The quantum well 702, the n-type semiconductor layer 706, and the p-type semiconductor layer 708 constitute a transverse waveguide 718. The transverse waveguide 718 is bounded on the n-side 724 in the Y direction by a lower-index n-cladding layer 710 and on the p-side 726 by a lower-index p-cladding layer 712. The n-side 724 and the p-side 726 are identified by parentheses. The DCICL 714 is a p-contact layer and is disposed on the p-cladding layer 712. The n-contact 716 is located below the substrate 704. The cavity 740 is orthogonal to the transverse waveguide 718 and is bounded in the Z direction at a first end 720 by an HR facet (not shown) and at a second end 722 by a PR facet 744. The dashed line demarcates the current injection boundary of the cavity 740 in the longitudinal direction defined by the p-contact DCICL 714.

[0041] In the present example, the p-contact layer of the DCICL 714 is configured to vary the amount of current injected into the cavity 740 in the longitudinal (or Z) direction by changing the surface area in contact with the p-cladding layer 712. The DCICL 714 includes a plurality of individual contacts 714a - 714k of uniform thickness. The contact size gradually decreases from contact row 714a to contact row 714k. In the example, the smaller contact sizes (e.g., in contact rows 714g - 714k) provide higher resistance and lower injection current density compared to the larger contacts (e.g., in contact rows 714a - 714f). Each contact of the DCICL 714 (contact rows 714a - 714k) may include a highly doped contact material (e.g., p++GaAs or p++GaN) that is coupled to a moderately doped underlying p-cladding layer 712 (e.g., p+GaAs) in order to facilitate graded current injection from the lowest current injection region at or near the PR facet 744 at the end 722 to the highest current injection region at or near the HR facet at the first end 720. By controlling the varying resistance along the contact layer DCICL 714, it is possible to inject more current near the HR facet than near the PR facet 744. This is partly due to the fact that the resistance of the smaller contacts at the end 722 (e.g., in contact rows 714g - 714k) is higher than the resistance of the larger contacts at the end 720 (e.g., in contact rows 714a - 714f). In some examples, the highly doped contact DCICL 714 may have a greater than about 1020 cm -3 The p-dopant concentration of the p-cladding layer 712 can be moderately doped, and the p-dopant concentration ranges from about 10 16 cm -3 to 10 18 cm -3 . In other examples, the highly doped p-contact DCICL 714 can have a p-dopant concentration greater than about 10 18 cm -3 , and the corresponding p-cladding layer 712 can be moderately doped, and the p-dopant concentration ranges from about 10 14 cm -3 to 10 16 cm -3 . In different examples, the highly doped p-contact DCICL 714 can have a p-dopant concentration greater than about 10 19 cm -3 , and the corresponding p-cladding layer 712 can be moderately doped, and the p-dopant concentration is less than about 10 19 cm -3 . The p-dopant concentrations in the P-contact layer DCICL 714 and the corresponding p-cladding layer 712 will vary with the design constraints of the particular laser diode 700 and will depend on various factors such as the substrate, dopant, desired power conversion efficiency (PCE), and / or free carrier loss tolerance. However, in order to effectively grade the current injection through the DCICL 714 in the laser diode 700, the p-dopant concentration in the p-cladding layer 712 should be less than the p-dopant concentration in the DICCL 714 layer.

[0042] In an alternative example, the thickness of each contact in the contact rows 714a - 714k can also be changed in order to further increase the current injection grading by more sharply grading the resistance of the DCICL 714 in the cavity 740 in the longitudinal direction. For example, as Figure 7B shown, the thickness of the smaller surface area contact 714k can be greater than the thickness of the larger contact 714a to increase the resistance of the smaller contact 714k with respect to the resistance of the larger contact 714a.

[0043] In an example, the p-dopant used in the DCICL 714 and / or the p-cladding layer can include nitrogen (N), phosphorus (P), boron (B), beryllium (Be), zinc (Zn), chromium (Cr), silicon (Si), germanium (Ge), etc. or any combination thereof. A wide range of patterns and manufacturing processes (e.g., mesa etching, cap etching, regrowth, or implantation) can be inscribed to achieve the appropriate current grading, and the claimed subject matter is not limited in this regard.

[0044] Figure 8AFIG. 0 is a cross-sectional perspective view showing a vertical epitaxial layer structure of an exemplary laser diode 800 including an n-type differential current injection contact layer (DCICL) 816. In the example, the laser diode 800 is formed to include a substrate 804, an n-type semiconductor layer 806, and a p-type semiconductor layer 808. An active layer (quantum well 802) is disposed between the n-type semiconductor layer 806 and the p-type semiconductor layer 808. An n-cladding layer 810 is disposed outside the n-type semiconductor layer 806. A p-cladding layer 812 is disposed outside the p-type semiconductor layer 808. The quantum well 802, the n-type semiconductor layer 806, and the p-type semiconductor layer 808 constitute a portion of a transverse waveguide 818 of the laser diode 800. The transverse waveguide 818 is bounded on the n-side 824 in the Y direction by the lower-index n-cladding layer 810 and on the p-side 826 by the lower-index p-cladding layer 812.

[0045] In the current example, the DCICL 816 is an n-contact layer and is disposed on the substrate 804. A p-contact 814 is disposed on the p-cladding layer 812. A cavity 840 defined by the p-contact layer 814 is orthogonal to the transverse waveguide 818 and is bounded in the longitudinal or Z direction at a first end 820 by an HR facet (not shown) and at a second end 822 by a PR facet 844.

[0046] In the example, the n-contact of the DCICL 816 is configured to vary the amount of electrical current injected into the cavity 840 in the longitudinal direction. The current injection can be varied by patterning the n-side metal contact of the DCICL 816 to vary the resistance of the DCICL 816, thereby injecting current with a longitudinal current density profile and with a higher current density near the HR facet at the first end 820 of the laser diode compared to the current injection at the second end 822 of the laser diode near the PR facet 844.

[0047] Figure 8B FIG. 10 is a plan view showing the laser diode 800 having an n-contact DCICL 816 with an alternative n-side metal pattern. Similar to Figure 8A the n-metal contact pattern shown in FIG. 12, more metal contact material is patterned near the HR facet at the end 820 and gradually decreases along the longitudinal (Z) direction. Dashed lines 850a-b show the perimeter of the p-contact 814. The width of the opening toward the PR facet 844 at the end 822 can match, or can be wider or narrower, compared to the width of the p-contact 814.

[0048] The above method may result in some inefficiencies in the overall power conversion because some power is dissipated within the parasitic contacts or within the device.

[0049] Figure 9AFIG. 0 is a cross-sectional view showing a vertical epitaxial layer structure of an exemplary laser diode 900 that includes a plurality of electrically isolated p-contacts 914a, 914b, 914c, and 914d configured to vary current density along a longitudinal cavity. In the example, the laser diode 900 is formed to include a substrate 904, an n-type semiconductor layer 906, and a p-type semiconductor layer 908. An active layer (quantum well 902) is disposed between the n-type semiconductor layer 906 and the p-type semiconductor layer 908. An n-cladding layer 910 is disposed outside the n-type semiconductor layer 906. A p-cladding layer 912 is disposed outside the p-type semiconductor layer 908. The quantum well 902, the n-type semiconductor layer 906, and the p-type semiconductor layer 908 constitute a portion of a transverse waveguide 918 of the laser diode 900. The transverse waveguide 918 is bounded on the n-side 924 in the Y direction by a lower-index n-cladding layer 910 and on the p-side 926 by a lower-index p-cladding layer 912.

[0050] In the present example, a plurality of electrically isolated p-contacts (MEIPs) 914a-d are disposed on the p-cladding layer 912. Each of the MEIPs 914a-d is coupled to a respective heat sink base 930a, 930b, 930c, and 930d, which are also segmented and electrically isolated. The bases 930a-d are mounted to a carrier 950. A cavity 940 defined by the MEIPs 914a-d is orthogonal to the transverse waveguide 918 and is bounded at a first end 920 in a longitudinal or Z direction by an HR facet 942 and at a second end 922 by a PR facet 944.

[0051] In the example, the MEIPs 914a-d are configured to vary the amount of electrical current injected into the cavity 940 in a longitudinal direction. Current injection can be varied by electrically isolating the MEIPs 914a-d and injecting a varying amount of current through each respective MEIP 914a-d (alternatively, two or more of the MEIPs 914a-d can contribute the same or a similar amount of current injection). Alternatively, current injection can be varied via the MEIPs 914a-d by fabricating two or more of the MEIPs 914a-d with different resistive characteristics or by fabricating two or more of the MEIPs 914a-d with different thicknesses. Figure 9B An example is shown of MEIPs 914a-d having different thicknesses T1 and T2, respectively; the bases 930a-b can be of different heights to accommodate the variable thicknesses of the MEIPs 914a-b.

[0052] In an example, the MEIPs 914a-d can be divided along the longitudinal direction of the cavity 940 and the current density can vary between the MEIPs 914a-d such that more current is injected via the MEIPs 914a-b than via the MEIPs 914c-d, thereby injecting current with a discrete longitudinal current density profile and having a higher current density near the HR facet at the first end 920 compared to lower current injection at the second end 922 near the PR facet 944 of the laser diode 900. It may be desirable to minimize the spectral width of the maximum current density in each segment for a given front and back length. The current density in each segment is adjusted to keep the "canonical temperature" approximately the same, which will result in a minimum spectral width. The degree of the current density also varies according to the number of segments. Alternatively, multiple individual contacts can be made to the laser and the current density along the cavity can be varied. This technique is not affected by an efficiency penalty. More or fewer MEIPs 914 than shown in the current example can be used and the claimed subject matter is not limited in this regard.

[0053] A variety of materials and methods can be used to fabricate the reference Figure 6 to the laser diodes 600, 700, 800, and 900 described with respect to FIGS. 6-9. For example, the substrates 604, 704, 804, and 904 can include gallium arsenide (GaAs), indium phosphide (InP), etc. or any combination thereof. The n-type semiconductor layers 606, 706, 806, and 906, the p-type semiconductor layers 608, 708, 808, and 908, the n-cladding layers 610, 710, 810, and 910, and / or the p-cladding layer 612 can be grown on the respective substrates 604, 704, 804, and 904 and include a variety of materials including any of the following: indium (In), gallium (Ga), aluminum (Al), arsenic (As), phosphorus (P), platinum (Pt), gold (Au), nickel (Ni), gallium arsenide (GaAs), indium phosphide (InP), aluminum gallium arsenide (AlGaAs), indium gallium arsenide phosphide (InGaAsP), indium gallium arsenide (InGaAs), etc. or any combination thereof. The n-type and p-type layers can be doped with dopants to produce the desired n-type or p-type material. Similarly, a variety of suitable deposition processes can be used to fabricate the laser diodes 600, 700, 800, and 900, including chemical vapor deposition (CVD), metalorganic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE).

[0054] The laser diodes 600, 700, 800, and 900 are provided above for illustrative purposes and do not include an exhaustive recitation of all the various methods and materials that can be used to fabricate such laser diodes. For example, there are a variety of alternative or additional ways to arrange the disclosed laser diodes including differential current injection contact layers. Such other example laser diodes can have more Figure 6More or fewer epitaxial layers, different epitaxial layer arrangements, and / or different numbers of features, structures, or combinations of structures and / or features as described in FIGS. 1 to 9 are intended to be within the scope of the present disclosure. Accordingly, the claimed subject matter is not limited in this respect.

[0055] The general and specific principles of examples of the presently disclosed techniques have been described and illustrated, and it will be apparent that the examples may be modified in arrangement and detail without departing from those principles. We claim all modifications and variations coming within the spirit and scope of the following claims.

Claims

1. A laser diode, comprising: A transverse waveguide, the transverse waveguide including an active layer between an n-type semiconductor layer on the n-side of the transverse waveguide and a p-type semiconductor layer on the p-side of the transverse waveguide, wherein the transverse waveguide is bounded on the n-side by an n-cladding layer and on the p-side by a p-cladding layer, the n-cladding layer having a lower refractive index than the n-type semiconductor layer, and the p-cladding layer having a lower refractive index than the p-type semiconductor layer; A cavity, the cavity being orthogonal to the transverse waveguide, wherein the cavity is bounded in the longitudinal direction at a first end by a high reflector (HR) facet and at a second end by a partial reflector (PR) facet; and A first contact layer configured to vary the amount of current injected into the cavity in the longitudinal direction so as to inject more current at the first end than at the second end, thereby reducing the longitudinal thermal gradient along the cavity, wherein the first contact layer includes a material thickness gradient such that the material thickness of the first contact layer increases longitudinally from the first end to the second end.

2. The laser diode according to claim 1, wherein, The first contact layer is disposed on the n-side of the transverse waveguide.

3. The laser diode according to claim 1, wherein, The first contact layer is disposed on the p-side of the transverse waveguide.

4. The laser diode according to claim 1, wherein, The first contact layer includes a contact material surface area gradient such that the contact material surface area of the first contact layer decreases longitudinally from the first end to the second end.

5. The laser diode according to claim 4, wherein The first contact layer includes a dopant.

6. The laser diode according to claim 5, wherein, The first contact layer is highly doped.

7. The laser diode according to claim 5, wherein The n-cladding layer below the first contact layer is moderately doped.

8. The laser diode according to claim 5, wherein, The p-cladding layer below the first contact layer is moderately doped.

9. The laser diode according to claim 4, wherein, The contact material surface area gradient includes a contact pattern.

10. The laser diode according to claim 9, wherein, The contact pattern includes a plurality of discrete contacts and is configured to grade current injection by varying the contact surface area size between different ones of the plurality of discrete contacts, wherein the surface area of the discrete contacts decreases longitudinally from the first end to the second end.

11. The laser diode according to claim 9, wherein, The contact pattern includes a continuous shape configured to gradually reduce the contact material surface area distribution longitudinally from the first end to the second end.

12. The laser diode according to claim 1, wherein, The first contact layer includes a metal alloy having reduced conductive properties.

13. The laser diode according to claim 4, wherein, The laser diode further includes a second contact layer disposed opposite the first contact layer and including a different contact pattern.

14. A laser diode, comprising: A transverse waveguide, the transverse waveguide including an active layer between an n-type semiconductor layer on the n-side of the transverse waveguide and a p-type semiconductor layer on the p-side of the transverse waveguide, wherein the transverse waveguide is bounded on the n-side by an n-cladding layer and on the p-side by a p-cladding layer, the n-cladding layer having a lower refractive index than the n-type semiconductor layer, and the p-cladding layer having a lower refractive index than the p-type semiconductor layer; A cavity, the cavity being orthogonal to the transverse waveguide, wherein the cavity is bounded in the longitudinal direction at a first end by a high reflector (HR) facet and at a second end by a partial reflector (PR) facet; A first p-side contact and a second p-side contact, both electrically isolated from each other, wherein the first p-side contact and the second p-side contact are electrically coupled to the cavity and the n-side contact, wherein the first p-side contact is configured to inject a first current density into the cavity, and the second p-side contact is configured to inject a second current density into the cavity.

15. The laser diode according to claim 14, wherein, The first current density and the second current density are different.

16. The laser diode according to claim 14, wherein, The first p-side contact and the second p-side contact are uniformly distributed along the cavity.

17. The laser diode according to claim 16, wherein, The first p-side contact is disposed close to the first end, the second p-side contact is disposed close to the second end, and the first current density is greater than the second current density.

18. The laser diode according to claim 14, wherein, The first p-side contact and the second p-side contact are arranged to be closer to the first end than the second end, such that when active, more current is injected into the cavity at a position closer to the first end compared to the second end.

19. The laser diode according to claim 14, wherein, The laser diode further includes a third p-side contact and a fourth p-side contact that are electrically isolated from each other and electrically coupled to the cavity and the n-side contact.

20. The laser diode according to claim 14, wherein, The first p-side contact and the second p-side contact are coupled to respective ones of a first heat sink base and a second heat sink base.

21. The laser diode according to claim 14, wherein, The first p-side contact and the second p-side contact have different thicknesses.

22. The laser diode according to claim 14, wherein, The first p-side contact and the second p-side contact include materials having different resistance characteristics.

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